Eastern Sierra Snowpack Summary - 2016-01-12 07:34

THIS ADVISORY EXPIRED ON January 14, 2016 @ 7:34 amAvalanche Advisory published on January 12, 2016 @ 7:34 am

Issued by
Doug Lewis - Eastern Sierra Avalanche Center

Avalanche Character 1: Wind Slab

Wind Slab avalanches release naturally during wind events and can be triggered for up to a week after a wind event. They form in lee and cross-loaded terrain features. Avoid them by sticking to wind sheltered or wind scoured areas.

Recent moderate winds have created isolated wind slabs on most aspects at mid to upper elevations, especially just below ridgelines and sides of gullie. The unconsolidated snow from the previous week continues to be easily transported, and the next storm system is forecast to bring additional strong SW winds, which will produce new windslabs on NW/NE/E/SE aspects. While these are sensitive to human triggering, the avalanches are not likely big enough to bury a person, but could be enough to trigger deeper releases in shallow snowpacks or take a ride into hazardous terrain. Older wind slabs that formed a few days ago are likely to exist in similar locations, and may still be sensitive to human triggering.

Avalanche Character 2: Deep Slab

Deep Slab avalanches are destructive and deadly events that can release months after the weak layer was buried. They are scarce compared to Storm or Wind Slab avalanches. Their cycles include fewer avalanches and occur over a larger region. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Avoid the terrain identified in the forecast and give yourself a wide safety buffer to address the uncertainty.

A weak basal snowpack still exists throughout much of the forecast area due to shallow early season snow conditions and cold temperatures. While it is unlikely for a human to trigger an avalanche at this level, the resulting slide would likely be large with bad consequences. This layer is becoming less and less of a concern as the snowpack deepens. However, it is still something to be aware of, especially where shallower snowpack exists outside of the Mammoth area. Regularly reevaluate the local snow stability by digging snowpits and performing stability tests to help you make informed decisions.

Snowpack Discussion

Wednesday 1/13 8am update: High winds at all elevations today - look out for sensitive winds slabs at all elevations in wind deposited areas!

The recent series of storms (Jan 4-7) deposited between 1 to 3 feet of low-density snow at the higher elevations with Mammoth Lakes Basin receiving the most snow this past week. Elsewhere amounts tapered off north of June Lake and south of the Mammoth Lakes Basin. Remote snow depth sensors indicate significant settlement ranging between 2 to 5 inches, signs the new snow is bonding and strengthening. The big concern is windslabs, which formed as the storms exited the region where strong upper elevation winds (Sat. W-NW, Mon. N-NE) produced windslabs throughout mid to upper elevations on most aspects. Recent stability tests suggest that overall the recent new snow is bonding well with old snow surface with the exception of freshly formed windslabs deposited over low-density snow. They’re widely distributed and difficult to generalize their location and sensitivity. Stability tests indicate they’re prone to triggered release, if encountered on steeper terrain (35 degrees or steeper). To further complicate matters, a storm system forecast to move though the region Tuesday night into Wednesday will bring another round of strong winds and likely form new windslabs on NW-E-SE aspects in favored locations (shallow depressions, leeward side of terrain features - i.e., rock ribs, tree fingers, convex rolls, ridgetops, etc.). While these windslabs are capable of producing avalanches, they are unlikely be large enough to bury a person. However, they could force a rider into undesirable terrain or potentially trigger deep releases. In areas north of June Lakes and south of Mammoth Lakes Basin, while unlikely, larger windslab release could possibly trigger the basal facets that have been consistently observed throughout the region, especially where the snowpack has been consistently shallow, producing a much larger release with severe consequences. While not a widespread concern, it should be factored in when deciding how and when to ski steeper slopes, especially where objective hazards are a concern. Possible trigger points include shallow areas such as around rock-outcrops or areas that have had bee wind striped. Backcountry travelers should anticipate that pockets of instability maybe encountered and adjust their objectives as conditions change.

Snow stability test results vary greatly dependent on location. Shallow snowpack regions (Virginia Lakes, areas south of Mammoth Lakes Basin, etc.) continue to show weaknesses between the new/old snow interface as well as recent windslabs over low-density snow on N/NE/E/SE aspects. Additionally, the persistent deep facets continue to fail when stressed. Areas where the snowpack is deeper overall (Mammoth Lakes Basin, mid-upper elevations surrounding June Lakes) the deep facets have gained some strength and are less reactive. Here, larger triggers are needed for failure (i.e., cornice drop, windslab avalanches stressing the lower snow structure resulting in failure). This is unlikely but possible, normal backcountry precautions (one person on the slope at a time, move from safe zone to safe zone) will reduce the stress on the snowpack and limit group exposure. Windslabs continue to be the overriding concern throughout the region as the recent storms exited the state, north winds followed resulting in a deterioration of ski conditions (semi-supportive wind crust) mixed with soft snow. Windslabs can be found on N/E/SE/S aspects and highly variable in extent and strength with some fully supportive while others not able to support a rider. The windslabs have been deposited over low-density snow and have proven to be very poorly bonded to the snow below with failures while isolating a column for testing. Ski cuts did not result in any activity but the possibility of a rider-triggered failure is certainly possible on terrain 35 degrees and steeper. Recently loaded convex rolls, drifts, and hollow sounding surface slabs should be approached with caution.

CURRENT CONDITIONSWeather Observations Between June (10,000 ft.) and Mammoth (11,000 ft.)

Tuesday – Wednesday: the High pressure ridge which has dominated the region recently will begin to move slowly east across Nevada, keeping the region dry with increasing southwest winds at the higher elevations and increasing high clouds (Cirrus) spreading across the region. As the high-pressure ridge exists the region, it opens the door for Pacific storm systems to move into to California. While the next storm system will primarily impact the Sierras north of Bridgeport, it ushers in a more active pattern for remainder of the week. The biggest impact for this first storm system for the eastern Sierras are the forecast winds as the upper level jet stream moves across the mountains with Southwesterly winds of 35 to 40 mph with gusts to 60 mph increasing to 55 to 60 mph with gusts to 95 mph late Tuesday night thru Wednesday.

Thursday – Friday: another shortwave moves quickly across the Pacific Northwest and into northern California. The storm track again favors northern California but light precipitation is forecast for the Sierra Crest. As the persistent upper level Low Pressure system, which has been parked over the northeastern U.S. begins to move to the east, this will allow the recent blocking pattern (High Pressure) to breakdown and move into the interior US with a more progressive pattern (aka stormy weather) in its wake for the weekend and into next week.

Two-Day Mountain Weather ForecastProduced in partnership with the Reno NWS

For 8,000 ft. to 10,000 ft.

Tuesday

Tuesday Night

Wednesday

Weather:

PARTLY CLOUDY.

MOSTLY CLOUDY. SLIGHT CHANCE OF SNOW AFTER MIDNIGHT.

CLOUDY. CHANCE OF SNOW IN THE MORNING...THEN SNOW IN THE AFTERNOON.

Temperatures:

34 TO 41 deg. F.

19 TO 25 deg. F.

30 TO 36 deg. F.

Wind direction:

SOUTHWEST

SOUTHWEST

SOUTHWEST

Wind speed:

10 TO 15 MPH WITH GUSTS TO 25 MPH.

35 TO 40 MPH WITH GUSTS TO 60 MPH INCREASING TO 40 TO 50 MPH WITH GUSTS TO 75 MPH AFTER MIDNIGHT.

40 TO 50 MPH WITH GUSTS TO 75 MPH.

Expected snowfall:

0 in.

0 in.

1 TO 4 INCHES in.

Over 10,000 ft.

Tuesday

Tuesday Night

Wednesday

Weather:

PARTLY CLOUDY

MOSTLY CLOUDY

CLOUDY. SNOW.

Temperatures:

32 TO 38 deg. F.

15 TO 21 deg. F.

26 TO 32 deg. F.

Wind direction:

WEST

SOUTHWEST

SOUTHWEST

Wind speed:

10 TO 15 MPH SHIFTING TO THE SOUTH IN THE AFTERNOON. GUSTS UP TO 30 MPH.

35 TO 40 MPH WITH GUSTS TO 60 MPH INCREASING TO 55 TO 60 MPH WITH GUSTS TO 95 MPH AFTER MIDNIGHT.

50 TO 60 MPH WITH GUSTS TO 95 MPH.

Expected snowfall:

0 in.

0 in.

1 TO 4 INCHES in.

Disclaimer

This snowpack summary applies only to backcountry areas outside established ski area boundaries. This snowpack summary only describes general avalanche conditions and local variations always occur. This snowpack summary expires 24 hours after the posted time unless otherwise noted. The information in this snowpack summary is provided by the Eastern Sierra Avalanche Center who is solely responsible for its content.

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